In the field of box-making, sheets, typically corrugated boards, are sequentially conveyed along a horizontal path to one or more stations along the path where operations like cleaning, printing, cutting, slotting or scoring are performed on the boards in a timed sequence. It is essential that the boards arrive at each of the aforementioned work stations in “registration”, that is, in a predetermined timed sequence. Various examples of corrugated board conveyors including timed feeders may be found in U.S. Pat. Nos. 4,045,015; 4,494,745; 4,632,378; 4,681,311; 4,889,331; 5,184,811 and, 7,635,124 B2.
Several methods of conveying the boards to the various stations along the path are presently in use in the industry. One uses opposed pull rolls which pull the boards through the nip between the rolls. Another method uses rotatable friction rolls made, for example, with a urethane surface on which the boards are maintained by vacuum. This method which is disclosed in U.S. Pat. Nos. 7,096,529 B2, and 5,004,221, is sometimes referred to as “vacuum transfer”.
Another vacuum transfer method employs a belt conveyor which supports the boards while they are held on the conveyor belt by vacuum. This type of conveyance is sometimes referred to as a “vacuum belt conveyor”, and one example of such is disclosed in U.S. Pat. No. 5,163,891.
The above methods have been and still are satisfactory where the boards are printed by passage between opposed rolls or cylinders, one being an “impression” roll and the other, a “print” roll having a printing plate and ink to transfer the image of the plate to the board in well-known fashion. However when a digital printer is used instead of the above system, a problem may arise when the boards are conveyed to the printer by a vacuum belt conveyor. In one form of this system, a vacuum transfer unit is used and the conveyor belt is perforated to provide a plurality of holes or apertures that communicate the vacuum with the board to hold the board on the belt. If any of the belt apertures adjacent to the edges of the boards is not covered or closed by the board, ink is subject to deviation (“windage”) from its intended position on the image being printed on the board. It is understood that the digital printer includes a print head having a plurality of ink discharge ports or nozzles from which the inks are deposited to form the image on the board. If the vacuum used to hold the boards on the conveyor belt is free to divert the flow of ink from the print head to the board to form the desired image, the resulting image will be adversely affected—smudged, distorted, off-color, etc. Such a result is of course not acceptable in the printing industry.